The present invention relates to Darrieus turbines and more particularly to an improved apparatus for controlling the level of output power produced by such a turbine as the fluid velocity varies.
Because of the demands for alternate sources of energy, increasing attention has been focused on wind power as an energy source. Vertical axis wind turbines have received particular attention because of their structural stability and their advantages over conventional windmills. In this regard, these advantages include the fact that no steering mechanism is required and that no heavy gear box and/or generator is needed at the top of the supporting tower.
Two principal types of vertical axis wind turbines are the Sovonius rotor and the Darrieus rotor. The two-bladed Savonius rotor basically comprises a pair of opposing half cylinders which are offset from the vertical support axis to form an S-shaped configuration, and uses the difference between wind forces acting against the concave and convex surfaces to generate power. Examples of Savonius rotors or modifications thereof are disclosed, for example, in U.S. Pat. Nos. 4,117,009 (Baum, Sr. et al.) and 4,117,014 (Kephart, Jr.).
The Darrieus rotor conventionally comprises at least one thin ribbon-type half-hoop, the chord of each half-hoop defining the axis of rotation. The cross-sectional profile of the continuous ribbon forming each half-hoop is that of a stream-lined airfoil. For a vertical axis wind turbine, each half-hoop attached to the central shaft defines the vertical axis of rotation with the blunt leading edge oriented so as to face into the relative air flow created by rotation of the central vertical shaft. The basic Darrieus rotor is described, for example, in U.S. Pat. No. 1,835,018 (Darrieus) and further modifications thereof are described, for example, in U.S. Pat. Nos. 4,082,479 (Rangi et al.) and 4,112,311 (Theyse).
As will appear, the present invention is concerned with turbines of the Darrieus type, and modifications thereof, as well as with related vertical axis wind turbines. A problem associated with vertical axis wind turbines concerns the operation thereof at high wind speeds. Specifically, during normal operation, the power output of the turbine increases as the wind speed increases until inherent stall conditions are achieved. This generally results in turbine ratings that are higher than desired and in higher energy costs, since the costs associated with the power conversion machinery are a function of the rating.
The Rangi et al. patent (U.S. Pat. No. 4,082,479) referred to above is concerned with preventing overspeeding of the rotor of a Darrieus turbine in high winds so as to prevent damage to the turbine and to this end, provides for mechanical spoilers in the form of relatively thin blade shaped spoiler elements which are attached to the airfoil section and which, due to centrifugal force, assume a position generally transverse to the air flow direction so as to increase drag and control rotor overspeeding. U.S. Pat. No. 4,115,032 of Lange discloses a drag-type turbine wherein a braking effect is created at high wind speeds to combat overspeeding. U.S. Pat. No. 4,117,009 of Baum discloses a Savonius-type system wherein a centrifugally actuated spoiler door section which opens at wind speeds above a predetermined level to dump air out of the rotor.